In cases such as JFK's assassination, why do so many people refuse to accept the simple and obvious conclusion that Oswald did it? Psychology.

With the passing of the 50th anniversary of the assassination of President John F. Kennedy last week, and the accompanying fusillade of documentaries purporting to prove there was a conspiracy behind it, we might expect (and hope) that cabalistic conjecturing will wane until the next big anniversary.

Don't count on it. A poll this month found that 61% of Americans who responded still believe that JFK was the victim of a conspiracy, despite the fact that the preponderance of evidence points to Lee Harvey Oswald as the lone assassin.

Why do so many people refuse to accept this simple and obvious conclusion? The answer: psychology.

There are three psychological effects at work here, starting with "cognitive dissonance," or the discomfort felt when holding two ideas that are not in harmony. We attempt to reduce the dissonance by altering one of the ideas to be in accord with the other. In this case, the two discordant ideas are 1) JFK as one of the most powerful people on Earth who was 2) killed by Lee Harvey Oswald, a lone loser, a nobody. Camelot brought down by a curmudgeon.

That doesn't feel right. To balance the scale, conspiracy elements are stacked onto the Oswald side: the CIA, the FBI, the KGB, the Mafia, Fidel Castro, Lyndon Johnson and, in Oliver Stone's telling in his film "JFK," the military-industrial complex.

Cognitive dissonance was at work shortly after Princess Diana's death, which was the result of drunk driving, speeding and no seat belt. But princesses are not supposed to die the way thousands of regular people die each year, so the British royal family, the British intelligence services and others had to be fingered as co-conspirators.

By contrast, there is no cognitive dissonance for the Holocaust — one of the worst crimes in history committed by one of the most criminal regimes in history.

A second psychological effect is the "monological belief system," or "a unitary, closed-off worldview in which beliefs come together in a mutually supportive network," in the words of University of Kent researchers Michael J. Wood, Karen M. Douglas and Robbie M. Sutton in a 2012 paper titled "Dead and Alive: Beliefs in Contradictory Conspiracy Theories." A conspiracy theory, they wrote, is "a proposed plot by powerful people or organizations working together in secret to accomplish some (usually sinister) goal." Once you believe that "one massive, sinister conspiracy could be successfully executed in near-perfect secrecy [it] suggests that many such plots are possible."

With this cabalistic paradigm in place, conspiracies can become "the default explanation for any given event." For example, the Kent researchers found that people who believe that Princess Diana was killed by MI6 were also more likely to believe that the moon landing was a hoax, that HIV was created in a laboratory as a biological weapon and that governments are hiding extraterrestrials. The effect is even more pronounced when the conspiracies contradict one another. People who believed that Diana faked her own death were "marginally more likely" to also believe that she was killed.

A third psychological effect is "confirmation bias," or the tendency to look for and find confirming evidence for what you already believe and to ignore disconfirming evidence. Once you believe, say, that 9/11 was an inside job by the Bush administration, you focus on the handful of anomalies that fateful day and connect them into a seemingly meaningful pattern, while ignoring the massive evidence pointing to Al Qaeda. JFK conspiracy theorists ignore the massive evidence pointing to Oswald while seeking deep meaning in trivial matters, such as the man with the umbrella on the grassy knoll, or the puff of smoke behind the picket fence, or the odd noises echoing around Dealey Plaza. Each become pregnant with meaning when the mind goes in search of cabals.

Such conspiracy-mongering may seem harmless, but there's a dark side. Another study this year by University of Kent researchers found that "exposure to information supporting conspiracy theories reduced participants' intentions to engage in politics, relative to participants who were given information refuting conspiracy theories." They attributed this effect to "feelings of political powerlessness." What can any of us regular folks do if the world is run by a handful of secret societies (like the Illuminati) or families (such as the Rockefellers or Rothschilds) or operatives (think CIA or KGB) operating clandestinely to establish a new world order?

What happens in history matters, and where conspiracies are real — as in the case of the assassination of President Abraham Lincoln — we should ferret them out. But when conspiracy-mongering leads to absurd conclusions and diverts our attention from real, pressing political issues and leads people to become politically apathetic, it can be a dangerous waste of time.

Michael Shermer is the publisher of Skeptic magazine, an adjunct professor at Chapman University and Claremont Graduate University and the author of "The Believing Brain."